A concave is a grate used to retard material for threshing and for separating grain from material other than grain. The concave is subjected to a lot of wear through abrasion. This abrasion is distributed unevenly across the concave.
U.S. Pat. No. 5,057,056 (Kambeitz 1991) addresses the problem of concave abrasion, by providing angled bars which direct incoming material away from the center of the concave, thereby distributing the wear more evenly.
U.S. Pat. No. 5,613,907 (Harden 1997) teaches the use of a concave having removable grids, which are easily replaceable as they become worn.